Mitogen-Activated Protein Kinase Cascades in Plant Innate Immunity - PAMP Signals in Plant Innate Immunity: Signal Perception and Transduction - page 357

regulated by the MPK3/MPK6 cascade. WRKY33 is a substrate of MPK3/MPK6.
It has been demonstrated that WRKY33 is phosphorylated by MPK3/MPK6 in vivo
in response to Botrytis cinerea infection in Arabidopsis . WRKY33 is required for
MPK3/MPK6-induced camalexin biosynthesis (Mao et al. 2007 ).
Nicotiana benthamiana WRKY8 transcription factor has been shown to be a
physiological substrate of the MAPKs, SIPK, NTF4, and WIPK (Ishihama et al.
2011 ). Clustered Pro-directed Ser residues, which are conserved in group 1 WRKY
proteins, in the N-terminal region of WRKY8 were phosphorylated by these MAPKs
in vitro. The interaction of WRKY8 with MAPKs depended on its D domain, which
is a MAPK-interacting motif, and this interaction was required for effective phos-
phorylation of WRKY8 in plants. Phosphorylation of WRKY8 increased its DNA
binding activity to the cognate W-box sequence (Ishihama et al. 2011 ). Ectopic
expression of WRKY8 induced defense-related genes. By contrast, silencing of
WRKY8 decreased the expression of defense-related genes and increased suscepti-
bility to the oomycete pathogen Phytophthora infestans and the fungal pathogen
Colletotrichum orbiculare (Ishihama et al. 2011 ). These results suggest that MAPK-
mediated phosphorylation of WRKY8 has an important role in triggering down-
stream immune responses.
The tobacco MAP kinase WIPK phosphorylates and activates NtWIF, a tran-
scription factor. The transgenic tobacco plants overexpressing NtWIF exhibited
constitutive accumulation of transcripts for PR genes, PR-1a and PR-2 (Waller et al.
2006 ). MPK3 phosphorylates a plant VirE2-interacting protein 1 (VIP1), a bZIP
transcription factor (Liu et al. 2010 ) . VIP1 is a direct target of the PAMP-induced
MPK3. Upon phosphorylation by MPK3, VIP1 relocalizes from the cytoplasm to
the nucleus and regulates the expression of the PR1 pathogenesis-related gene
(Djamei et al. 2007 ). Collectively, these results suggest that phosphorylation of
transcription factors by MAPKs is an important event in triggering expression of
defense-related genes.

7.20 MAPKs Regulate Defense Gene Expression

by Releasing Transcription Factors in the Nucleus

Transcription factor release may be a common theme after MAPK activation to
control downstream gene expression. MAPKs activate expression of defense-related
genes. MKS1 (for MAP kinase 4 substrate 1) is a substrate for the Arabidopsis
MAPK MPK4. MKS1 interacts with the transcription factors WRKY 25 and
WRKY33 (Andreasson et al. 2005 ). The interaction of MKS1 with WRKY33 has
been shown to be dependent on the phosphorylation status of MKS1 induced by
MPK4 (Qiu et al. 2008b ). In the absence of pathogens, inactivated MPK4 forms a
ternary complex with MKS1 and WRKY33 in the nucleus, which prevents WRKY33
from functioning as a transcription factor. MPK4 is activated by the PAMP treat-
ment or pathogen inoculation in Arabidopsis . Upon activation of MPK4, MKS1 is
phosphorylated by MPK4. Subsequently, phosphorylated MKS1 and WRKY33